The present invention relates to treatments by a device for induction heating of continuously moving metal strips for heat-treatment or coating-treatment purposes; the coating may be metallic (reflow of tin or zinc) or organic (stoving of enamel or baking of paint). Induction heating, because of its intrinsic qualities (cleanliness, rapidity, flexiblity) is a technique particularly well suited to the continuous treatment of metal strips. This is why induction heating is employed for the treatment of the coating, either metallic or organic coating, of metal strips, in particular steel strips.
In the case of the heating of magnetic steels, in particular, the inductors employed generate mainly a longitudinal magnetic field parallel to the direction of continuous movement of the metal strip. The operating frequency of the inductors depends on the electrical, magnetic and geometrical characteristics of the metal strip, as well as on the power density for magnetic materials. The inductors are constituted by a coil in the form of a flattened helix of several turns, supplied by a frequency converter.
Such an induction coil generates a magnetic field which includes a significant longitudinal component, but unfortunately it also creates a transverse component. The first, longitudinal component generates induced currents which flow in the thickness of the strip perpendicularly to the direction of continuous movement and which heat it. The transverse component generates so-called leakage currents which originate in the strip, initially travel along it in the direction of continuous movement of the strip and which seek to return to the point of departure, following all the pathways (roller, metal casing, etc.) possible.
If an insulator, for example a layer of paint or of enamel, is interposed in the path of these leakage currents, a voltage then appears, the level of which may cause the destruction of this insulator. This phenomenon, known by the name of the spark-erosion phenomenon, is particularly detrimental to the quality of the paints or enamels which are deposited on the metal strip.
As regards conductive coatings, that is to say metal coatings, there is no deterioration in the quality since the currents may flow freely; by contrast, they may damage certain components, such as the bearings of the rollers for guiding the strip, for example. As regards organic coatings, a highly insulated guiding system (roller mounted on a block or coated with a plastic) may be used.
For multipurpose treatment installations, the use of metal rollers is essential. In this latter case, it is possible to place collectors on the edges of the strip, upstream and downstream of the inductor, so as to localize the defects. Unfortunately, this system leads to the manufacture of a strip of lower quality since these edges are bared and this is not a highly reliable system since there is the risk of "solid skin" paint breakdowns.
The origin of the leakage currents is the tranverse component of the magnetic field created by the inductor. The latter has, in cross-section, the shape of an elongate rectangle surrounding the metal strip to be treated and includes straight turns which are joined together by an oblique connection arranged on the side of the turn.
It is desirable to eliminate the leakage currents in order to improve the performance characteristics of the coating-treatment installations and, in particular, to be able to use multi-purpose installations. This is why the present invention proposes to provide an inductor for induction heating continuously moving metal strips, which enables the undesirable leakage currents to be eliminated.